Kaolinite, commonly known as kaolin clay, is a naturally occurring mineral with the chemical composition Al2Si2O5(OH)4. Kaolin clay is one of the most common minerals on earth, but deposits of commercial size and purity are relatively rare.
Sedimentary kaolins generally are contaminated with titanium minerals to the extent of about 1% to 4%, expressed as TiO2. Titanium minerals in the clay fraction of sedimentary kaolin have been identified primarily as anatase, although small amounts of other minerals such as leucoxene and brookite also have been detected. These minerals are usually heavily stained by iron and as a result vary from yellow to dark brown in color.
Natural occurring kaolin clay deposits contain discoloring contaminants, such as iron and titanium minerals. Titanium minerals in kaolin usually occur as discolored particles and these, coupled with iron oxides and other ferriferous material, are largely responsible for the yellow-brown shade of many kaolins. Often a clay is rejected as being unsuitable for commercial use solely on the basis of brightness, even though its other physical properties, such as the viscosity of clay-water slurries and particle size, are within desired limits.
Much attention and research in the industry has focused on developing processes to increase the brightness of clays. In the water-wash process, crude clay is slurried, degritted, fractionated to the desired particle size and the resulting fractions leached with zinc or sodium dithionite at a pH of 3.5-4.0 to improve both brightness and shade. Brightness improvements due to dithionite leaching can be quite substantial; however, the increase in brightness is generally insufficient to make high-brightness products in the range of 90 GE brightness.
Therefore, other processing methods, such as selective sedimentation, magnetic separation, froth flotation, and selective flocculation, have been developed for use in conjunction with dithionite to improve the brightness of the leached products. These methods can be employed to produce both standard and high-brightness products from highly discolored starting materials by removing much of the iron-stained titanium and ferriferrous material prior to the leaching step. Selective sedimentation, magnetic separation, froth flotation, and selective flocculation are processes designed to remove the highly discolored titanium impurities and iron oxides. The dithionite leach step solubilizes and removes a portion of the amorphous ferriferrous impurities on the clay surface.
The most successful processes developed to date to improve the brightness of clays are those in which impurities, such as titanium and iron compounds, are removed by selective sedimentation, selective flocculation, more commonly referred to as Differential Flocculation of Anatase (DFA), and magnetic separation. All three processes produce a substantial amount of reject material which is often discarded.
There is a need in the art for improved processes that can produce high-quality, high-brightness kaolins, while at the same time, achieving high yield for good process economics. There is also a need for process adjustability and the ability to optimize the process depending on the feedstock, i.e. the source of kaolin reserves which can vary widely in impurities and other attributes.